1. Field of the Invention
This invention relates to liquid crystal display systems and particularly to such systems that form a full color image through an adjacent dot process.
2. Description of the Prior Art
The term liquid crystal is applied to substances whose rheological behavior is similar to that of fluids but whose optical properties are similar to the crystalline state. This mesomorphic behavior is fairly common in organic compounds. Both aliphatic and aromatic compounds can exist as liquid crystals.
There are three basic types of liquid crystal material. These are termed nematic, smectic and cholesteric. Nematic liquid crystals consist of rod-like organic molecules. These molecules can move in the direction of their long axis and from side to side but their many other mechanically possible motions are constrained by the forces between them. They maintain a parallel, or nearly parallel arrangement, although each molecule can rotate about its axis of length symmetry. The word nematic comes from the Greek work meaning thread. When a nematic liquid crystal material is viewed under a microscope, one sees tiny thread-like formations. Each molecule in the nematic liquid crystal material is constrained to maintain its longitudinal orientation with respect to the other molecules in its domain, however, it is free to rotate about its longitudinal axis. Nematic liquid crystal molecules are also free to move from side to side or in a lengthwise direction. Such a packing arrangement has been likened to the orientation of wooden matches in a match box; the matches can move but tend to remain in parallel relationship with respect to each other.
Smectic liquid crystal molecules array themselves in layers. Any two layers can slide each over the other because the molecules cannot move in the direction of their long axis but only foward and backward or side to side in layers. Like nematic molecules, smectic molecules can rotate freely about their direction of length alignment. The word smectic is derived from the Greek work for soap. Smectic molecules are arranged in layers that can slide over each other however the molecules cannot move from layer to layer. The smectic liquid crystal materials incorporate the contraints of the nematic liquid crystal materials and are burdened with the further constraint of being incapable of migrating from layer-to-layer.
Cholesteric liquid crystals bear some resemblance to both smectic and nematic liquid crystals. These molecules are smectic in that they are arranged in layers, however, the molecular pattern itself is nematic.
These substances and their properties have been further examined by Heilmeir et al in Proceedings of IEEE vol. 56, No. 7, July, 1968 pages 1162-1171 in a paper entitled DYNAMIC SCATTERING: A NEW ELECTRO-OPTIC EFFECT IN CERTAIN CLASSES OF NEMATIC LIQUID CRYSTALS. In another paper also authored by Heilmeir entitled LIQUID CRYSTAL DISPLAY DEVICES In Scientific American vol. 222, April, 1970 starting at page 102:
"A molecule has an electric dipole moment simply because it is an electric dipole, that is, it can be described as an object having two electric charges separated by a distance. In the case of a molecule one end tends to be positively charged and the other tends to be negatively charged because of the asymmetrical distribution of the electrons that bind the atoms in the molecule. In ordinary liquids the electric forces are not strong enough to keep the molecules aligned because of the molecules' natural tendency to move randomly and independently. As a result perhaps only one in a thousand molecules would be aligned by an electric field.
As an electric field is imposing order on a liquid crystal it can also set in motion a chain of events that disrupts the molecular pattern forming under its influence. Nematic solutions contain ionic impurities (positively or negatively charged molecular fragments that are not from the nematic compound) and other ions that are probably produced by dissociation of the nematic compound itself. The electric field pulls the ions towards one or the other of its poles. In a nematic substance such as PEBAB the dipole moment of a molecule lies along its structural axis, and in an electric field the axis of the molecules of the substance line up parallel to one another. Ions can pass through an array without creating large disturbances. The term PEBAB refers to the nematic liquid crystal p-ethoxy-benzylidene-p-aminobenzonitrile.
Suppose, however, that the permanent dipole moment does not lie along the structural axis of the molecules. Such a molecule is by no means rare, because many molecules have side chains of atoms attached to their main chain. The electric dipole moment is along these branches rather than along the "backbone" of the molecule.
One substance of this kind is the nematic liquid crystal anisylidene-p-aminophenylacetate (APAPA). When this substance is subjected to an electric field, its molecules line up not along their main chain but perpendicularly to it because of the strong influence of the side chains. The main chains are therefore oriented in various directions with respect to the electric field. Aligned in this way the molecules present a barrier to the moving ions. The ions push through the log jam and disrupt the array, creating comparatively large regions of turbulence (from one micron to five microns across). The turbulence causes the thin layer of nematic material, which was originally transparent, to become milky white because the turbulent fluid scatters light. This effect is called dynamic scattering. Dynamic scattering can be halted and the clarity of the liquid crystal restored simply by turning off the voltage.
Prior art three-color color display devices have utilized a shadow-mask color cathode ray tube. These devices have characteristically required a comparatively great amount of space. Power dissipation in prior art devices has been high and the voltage demands of such systems have been of the order of many kilovolts.
It would be of great benefit to the art to provide a full color flat panel liquid crystal display device which would dissipate much less power than the shadow-mask color cathode ray tube. It would also be of great benefit to the art to provide a display device which would operate on a much lower voltage level than the shadow-mask color cathode ray tube and be usable in the same applications. It would also be of great benefit to provide a replacement for the shadow-mask color cathode ray tube that would permit the fabrication of far more compact and rugged color television displays.